Among various high-integration nonvolatile memory devices, a flash electrically erasable programmable read-only memory (EEPROM) device has particular advantages of, for example, high programming speed and low electricity consumption. As a result they are used as a large-capacity storage medium for portable terminals such as a digital camera, portable personal computer, etc., or used in place of a conventional hard disc.
Flash EEPROM devices may be generally classified, on the basis of a memory cell structure, into NAND-structure devices and NOR-structure devices. The NAND-structure devices have superior characteristics in the view of an integration degree, whereas the NOR-structure devices have superior random access time characteristics. In the NOR-structure devices, each of memory cells is independently connected to a bit line and a word line, and therefore, there is an advantage in that each cell interferes with other cells less during a writing operation or reading operation thereof.
However, the NOR-structure devices require a contact for the inter-connection of each cell and a corresponding bit line. For this reason, the NOR-structure devices are disadvantageous in the view of an integration degree (i.e., decreasing size), as compared to the NAND-structure devices, which require only one contact per a single string, i.e. a single unit consisting of a plurality of cells connected to one another in series.